Battery replacement neccessary or is iDevice fully bricked?

Hello everybody, I got an iPhone 5s wich seems completly dead, black screen, no sign of live, no sound, no logo, whatever.

Here is how I got there:

- installed iOS 8.3 and JB, but battery behaviour decreases to 5-6h standby

- tried a couple of iOS settings and tested some days but nothing worked

- read about battery life problems when an update didn't went well, decided to reset iPhone

- because I didn't want to update to 8.4.1 I decided to reset the phone without itunes by the option

- this was first mistale because of the JB this ends in a bootloop

- wanted to fix this with itunes but only had a virtual machine windows where iTunes didn't recognize

- next day I tried to reset with mates native windows but phone appears to be dead now

I think the battery was drained empty over night, and because there was a bootloop where the phone permanently tried to flash the iOS I'm worried that I crashed the NAND memory.

I was at the genius bar but they only telling me to switch devices. My hope is that maybe it is the battery and changing it can fix the problem. logicboard replacement is no option for me.

anyone knows how to fix an iPhone with black screen? it doesn't worry about any button combo, several cables, iTunes over win or even macbook. it definetly can't be fixed by button tricks